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An Atlas of Stellar Spectra An Atlas of Stellar Spectra Astrophys. monographs, Univ. Chicago Press (1943) AN ATLAS OF STELLAR SPECTRA WITH AN OUTLINE OF SPECTRAL CLASSIFICATION Morgan * Keenan * Kellman Table of Contents INTRODUCTION THE O5-F2 STARS The O Stars O9.5 B0 B0.5 B1 B2 B3 B5 B8 The Spectrum of Draconis The A Stars B9 A0 A1 A2 A3 A5 A7 F0 F2 The Peculiar A Stars file:///E|/moe/HTML/ASS_Atlas/MK_contents.html (1 of 3) [10/27/2003 4:22:18 PM] An Atlas of Stellar Spectra The Metallic-Line Stars The Spectrum of Bootis THE F5-M STARS F5 F6 F8 G0 G2 G5 G8 K0 K2 K3 K5 The M Stars THE SUPERGIANTS OF CLASSES B8-M2 FIVE COMPOSITE SPECTRA CONCLUSION Plate 1: O5 - B0 Standards, Plate 2: Two Wolf-Rayet Stars, Plate 3: Main Sequence O9 - B9, Plate 4: Supergiants O9.5 - A0, Plate 5: Luminosity Effects at O9 Plate 6: Luminosity Effects at O9.5, Plate 7: Luminosity Effects at B0, Plate 8: AG Pegasi = BD 11 4673, Plate 9: Luminosity Effects at B0.5, Plate 10: Luminosity Effects at B1 Plate 11: Luminosity Effects at B2, Plate 12: The Be Stars and Phi Persei, Plate 13: The Bnn Stars, Plate 14: P Cygni, Plate 15: Luminosity Effects at B3 Plate 16: Luminosity Effects at B5, Plate 17: Luminosity Effects at B8, Plate 18: The Peculiar Stars Beta Lyr and Nu Sgr, Plate 19: Main Sequence B8 - A2, Plate 20: Supergiants A0 - F0 Plate 21: Luminosity Effects at A0, Plate 22: Main Sequence A2 - F0, Plate 23: The "Metallic-Line Star" 63 Tauri, Plate 24: Two Peculiar A Stars, file:///E|/moe/HTML/ASS_Atlas/MK_contents.html (2 of 3) [10/27/2003 4:22:18 PM] An Atlas of Stellar Spectra Plate 25: Luminosity Effects at A3, Plate 26: Luminosity Effects at A5, Plate 27: The Spectrum of 17 Leporis, Plate 28: Luminosity Effects at A7, Plate 29: Main Sequence F0 - M2, Plate 30: Supergiants F0 - K5 Plate 31: Luminosity Effects at F0, Plate 32: The Peculiar F Stars Beta CrB and Gamma Equ, Plate 33: The Cluster-Type Variable RR Lyrae, Plate 34: The Variable Star SU Tau, Plate 35: Luminosity Effects at F6 Plate 36: Normal Giants F8 - K5, Plate 37: Luminosity Effects at F8, Plate 38: High Luminosity Stars at F8, Plate 39: The Cepheid Variable X Cygni, Plate 40: Luminosity Effects at G0 Plate 41: Luminosity Effects at G5, Plate 42: The High-Velocity Star Boss 2527, Plate 43: The Variable Star V Vul, Plate 44: Luminosity Effects at G8, Plate 45: The High-Velocity Star Delta Leporis Plate 46: Luminosity Effects at K0, Plate 47: High Luminosity Stars at K1, Plate 48: Luminosity Effects at K3, Plate 49: High Luminosity Stars at K3, Plate 50: Luminosity Effects at K5 Plate 51: The M Giant Sequence, Plate 52: The M Sequence as a Temperature Sequence, Plate 53: Luminosity Effects in the Early M Giants, Plate 54: A Carbon Star and a Long Period Variable, Plate 55: The Banded Stars in the Visual Region file:///E|/moe/HTML/ASS_Atlas/MK_contents.html (3 of 3) [10/27/2003 4:22:18 PM] An Atlas of Stellar Spectra 1. INTRODUCTION The Atlas of Stellar Spectra and the accompanying outline have been prepared from the viewpoint of the practical stellar astronomer. Problems connected with the astrophysical interpretation of the spectral sequence are not touched on; as a consequence, emphasis is placed on ``ordinary'' stars. These are the stars most important statistically and the only ones suitable for large-scale investigations of galactic structure. The plan of the Atlas can be stated as follows: a) To set up a classification system as precise as possible which can be extended to stars of the eighth to twelfth magnitude with good systematic accuracy. The system should be as closely correlated with color temperature (or color equivalent) as is possible. The criteria used for classification should be those which change most smoothly with color equivalent. b) Such a system as described under (a) requires a classification according to stellar luminosity, that is, the system should be two-dimensional. We thus introduce a vertical spectral type, or luminosity class; then, for a normal star, the spectrum is uniquely located when a spectral type and a luminosity class are determined. The actual process of classification is carried out in the following manner: (1) an approximate spectral type is determined; (2) the luminosity class is determined; (3) by comparison with stars of similar luminosity an accurate spectral type is found. As it may not be immediately apparent why an increase in accuracy in spectral classification is desirable, a short digression on some problems of stellar astronomy will be made. The problem of stellar distribution in the most general sense does not require any spectroscopic data. Stars of all types and temperatures may be considered together, and some general features of the distribution of stars in the neighborhood of the sun can be found. For this purpose a certain frequency distribution of stellar luminosities must be assumed. This luminosity function has a large dispersion and must be varied with Galactic latitude. In addition, there are certain regional fluctuations in the frequency of stars of higher luminosity of classes B, A, and M. As a result of these considerations (and because of difficulties with interstellar absorption) the general method has very definite limitations; the large dispersion of the luminosity function means we must have a large sample, and this in itself precludes detailed analyses of limited regions. In addition, there is evidence of clustering tendencies for stars of certain spectral types - a cluster or star cloud might be well marked for stars of type A, for example, and be not at all apparent from a general analysis of star counts. There is, then, for certain kinds of problems a great advantage in the use of spectral types of the accuracy of the Henry Draper Catalogue. Consider, for example, the stars of classes B8-A0 as a group. The dispersion in luminosity is far less than in the case of the general luminosity function, and the space file:///E|/moe/HTML/ASS_Atlas/MK1.html (1 of 4) [10/27/2003 4:22:19 PM] An Atlas of Stellar Spectra distribution of stars of this group can be determined with a correspondingly higher accuracy. In addition, we are able to correct for systematic errors due to interstellar absorption from observations of the color excesses of these stars. We have thus gained in two particulars: we have limited at one time the dispersion in luminosity and in normal color. The further refinement of a two-dimensional classification makes possible an even greater reduction in the dispersion in absolute magnitude for groups of stars. The mean distance of a group of stars of the same spectral type and luminosity class can be determined with great precision, even when the group consists of a relatively small number of stars. Even for individual stars distances of good accuracy can be derived. A corresponding gain is made in problems concerned with intrinsic colors and interstellar absorption. In the fifty-five prints which make up the accompanying atlas an attempt has been made to show most of the common kinds of stellar spectra observed in stars brighter than the eighth magnitude. The dispersion selected is intermediate between that used for very faint stars, where only a few spectral features are visible, and the larger-scale slit spectra which show a multitude of details. A sufficient number of lines and bands are visible to allow an accurate classification to be made, both by temperature and by luminosity equivalent, while the relatively low dispersion makes it possible to observe bright and faint stars in a uniform manner and avoids the possibility of appreciable systematic differences in their classification. A small one-prism spectrograph attached to the 40-inch refractor was used to obtain the plates. The reduction of collimator to camera is about 7; this makes it possible to use a fairly wide slit and still have good definition in the resulting spectra. The spectrograph was designed by Dr. Van Biesbroeck and constructed in the observatory shop by Mr. Ridell. The camera lens was constructed by J.W. Fecker, according to the design of Dr. G.W. Moffitt. The usable spectral region on ordinary blue-sensitive plates is from the neighborhood of K to H ( 3920-4900). The dispersion used (125 A per mm at H ) is near the lower limit for the determination of spectral types and luminosities of high accuracy. The stars of types F5-M can be classified with fair accuracy on slit spectra of lower dispersion, but there is probably a definite decrease in precision if the dispersion is reduced much below 150 A per mm. The lowest dispersion capable of giving high accuracy for objective-prism spectra is greater; the limit is probably near 100 A per mm. The minimum dispersion with which an entirely successful two- dimensional classification on objective-prism plates can be made is probably near 140 A per mm. This value was arrived at from a study of several plates of exquisite quality taken by Dr. J. Gallo, director of the Astronomical Observatory at Tacubaya, Mexico; for plates of ordinary good quality the limit is probably considerably higher. The Atlas and the system it defines are to be taken as a sort of adaptation of work published at many observatories over the last fifty years. No claim is made for originality; the system and the criteria are file:///E|/moe/HTML/ASS_Atlas/MK1.html (2 of 4) [10/27/2003 4:22:19 PM] An Atlas of Stellar Spectra those which have evolved from a great number of investigations.
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